Optical reflecting telescopes are used in a majority of laser radar applications. The predominant design used as the transmitter and receiver antennas is the Cassegrain system or a modified form thereof. The Cassegrain optics achieve a relatively long effective focal length on a compact optical train. Having such a short system is an advantage when rapid slewing and accurate pointing and tracking are required.
Achieving optical isolation between the transmitted laser beam and the receiver detectors is accomplished in various ways. Very often, dual apertures are used; one for the transmitter and one for the receiver. Disadvantages with this system include the increased weight penalty of the dual aperture and the difficulties which arise in mutual alignment of the two optical axes. Systems using a single transmit-receive aperture are more compact, but switching complexities between transmit and receive modes are very costly. With the single convex secondary mirror in the single transmit-receive aperture system, the transmitted laser beam must be decollimated and focused before its entrance into the optical train. Optical isolation can be achieved using an optical switch or isolator with a somewhat increased cost penalty. However, both the decollimation and isolation problems become much more serious when using a high energy laser beam. With these obstacles in mind the described invention herein achieves a simple solution to the problems.